1. Field of the Invention
This invention relates to a sealed type fluid reservoir, and more particularly to a sealed type fluid reservoir having a valve member provided in a main cap of a casing and by which pressure in the interior of the casing is maintained at atmospheric level.
2. Description of the Prior Art
The pressure in the interior of the casing of the brake master cylinder, for instance, varies and becomes positive or negative in comparison with atmospheric level due to the variation of ambient temperature or quantity of brake fluid. In order to maintain the pressure in the interior of the casing at atmospheric level, there has been provided a sealed type reservoir with a pair of check valves, both of which are disposed to operate in opposite directions.
The typical check valve of the conventional sealed type fluid reservoir, for example as disclosed in U.S. patent application, Ser. No. 712,862, filed Aug. 9, 1976, comprises a radially outwardly extended flange or rod element secured to the main cap, a first lip member air-tightly engaged with one face of the flange and a second lip member air-tightly engaged with the other face of the flange. The lip is at least formed integrally with the main cap which is deformable due to pressure variation in the interior of the casing. Consequently, sealing pressure between the flange and the lip integrally formed with the main cap is changed. In order to open the check valve at a predetermined pressure of the interior of the casing, the quality of the main cap and the condition of the interference of the lip have to be taken into consideration in the design thereof.
Further it is difficult to air-tightly engage the lip with the flange in light of manufacturing errors. For this reason, it is difficult to design and manufacture such check valves.